Well, I'm not an expert... just passing on information my tri coach has given me.
The idea of long rides for base miles is a good one... and yes, a recovery ride will be good too... but you need a short, all out hard ride too.
I know for me, I only have 3 days a week to ride... so I can't waste any of my rides on a lazy, casual ride. Every mile has to count.
Here is what my coach sent to me... and it applies to cycling. We aren't focusing on my swimming or running... so this is for my cycling.
NOTE: I am training for an Ironman in August 2008. So, I will need to be able to swim 2.4 miles, ride 112 miles and then run a marathon at the end. And the bike course is hilly. Not extremly hilly, but hill none-the-less.
2. base training is done at 68-72% of max hr, or a 7 on the 1-10 scale of perceived effort. if you are mid-6 to very low 7, ok. this is the effort level for the majority of your training. even when we ratchet the intensity up, more than two-thirds of your training volume will be at this effort level. this is the effort level that your body learns to burn fat most efficiently, and you should feel like you can go at this effort all day, as long as you keep fueled. most folks go too hard on their easy days and too easy on their hard days--we want to avoid this mistake. when you do not see a specific intensity for a given workout on your schedule, this is the effort level at which you should be training.
3. tempo effort is 88%, or a high 8 on the 1-10 scale. we will start with short distances at this effort level and extend them. this effort is also called (incorrectly) your lactate threshold, or your anaerobic threshold. this is the effort where there is balance between the energy required for work and the muscular waste created as a result of that work and the ability of the body to supply that energy. above this effort is the red zone, wehre you cannot train for very long periods without ample recovery. tempo work is important for improving your performance at all race distances. it will always be a challenging workout.
4. workouts that include intensities above threshold will be fewer, but they do have a role. they maximize the efficiency with which you body uses the energy you currently have going to the cells. you do not want to go into this red zone during your longer races, as the time it takes you to recover from the effort will negate the effort itself.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"